A fuel cell may be divided into a fuel cell stack configured to generate electric energy, a fuel supply system configured to supply fuel (e.g., hydrogen) to the fuel cell stack, an air supply system including an air blower and a humidifier to supply oxygen in the air, an oxidizing agent, required for an electrochemical reaction to the fuel cell stack, and a heat and water management system configured to adjust an operation temperature of the fuel cell stack.
Recently, fuel cell vehicles equipped with fuel cells as a driving source of vehicles have been released and development of eco-friendly future vehicles has been ongoing. A fuel cell is a type of electricity generation device for generating electricity as a main energy source of fuel cell vehicles, having a structure in which an anode to which hydrogen is supplied and a cathode to which air is supplied are stacked with a membrane electrode assembly (MEA) interposed therebetween and oxygen in the air and hydrogen supplied from the exterior are chemically reacted to generate electric energy.
Thus, while the fuel cell is operated, hydrogen having high purity is supplied to the anode of the fuel cell and air in the air is simultaneously directly supplied to the cathode of the fuel cell using an air supply device such as an air blower to generate electric energy. Accordingly, hydrogen supplied to the fuel cell is separated into hydrogen ions and electrons in a catalyst of the anode, and the separated hydrogen ions are transferred to the cathode through the electrolyte membrane, and oxygen supplied to the cathode is bonded with electrons introduced to the cathode through an external conducting wire to produce water, thus generating electric energy.
The generated electric energy is used in a driving motor of a fuel cell vehicle while is equipped with the fuel cell drives. When the fuel cell vehicle is stopped and a parking state thereof is maintained for a predetermined period of time, the fuel cell is shut down and supply of air and hydrogen to the fuel cell is stopped. When the parking state of the fuel cell vehicle is maintained for a long period of time, hydrogen remaining in the anode passes through an electrolyte membrane to be transferred to the cathode, rendering pressure of the anode to be less than the cathode, which results in formation of negative pressure in the anode whose entrance and exit are blocked. Due to the negative pressure of the anode, a nitrogen component within the cathode is introduced to the anode, and thus, pressure of the anode is recovered to normal pressure.
As illustrated in FIG. 1, in the process of forming negative pressure and recovering normal pressure of the anode, as the anode is reduced, negative pressure rapidly proceeds, and the pressure is gradually increased from a point of a negative pressure peak (please refer to “P” of FIG. 1) to be recovered to normal pressure. When the parking state of the fuel cell vehicle continues even thereafter, oxygen within the cathode is introduced to the anode, forming an interface between hydrogen and oxygen (e.g., H2/air front) in the anode to cause deterioration of the fuel cell due to the H2/air front.
In addition, when the fuel cell vehicle is parked for a substantial period of time, a gas other than hydrogen is mixedly introduced to the anode. Thus, when the fuel cell vehicle is restarted after the long-term parking, a high potential is formed to severely deteriorate the fuel cell to degrade durability of the fuel cell stack. Thus, to prevent a degradation of the fuel cell until the fuel cell vehicle, which has been parked for a substantial period of time, is restarted, a fuel cell purging method of periodically/aperiodically supplying hydrogen to the anode to exhaust oxygen within the fuel cell has been introduced.
However, with the fuel cell purging method, when the full cell is shut down and started up, hydrogen purging is performed, and when a parking time has lapsed, additional hydrogen purging is periodically performed, increasing hydrogen consumption to degrade actual fuel efficiency of the fuel cell vehicle. Among other methods for preventing degradation of a fuel cell is a start up cathode oxygen depletion (SU COD) scheme to reduce a potential of a stack by temporarily connecting a resistor when a high potential is detected when a vehicle is started. Here, hydrogen is additionally supplied to the anode.
However, the SU COD scheme has a high possibility of a fatal degradation of fuel cell performance due to a reverse voltage generated in a cell of a fuel cell stack when fuel is supplied unevenly. In addition, since hydrogen is additionally supplied, hydrogen consumption is increased to degrade actual fuel efficiency of the fuel cell vehicle.